1. Field of the Invention
The present invention relates to a method and apparatus for roll changing on a winding device, and more particularly, to a device for severing a moving web and causing it to envelop a new core so that a winding operation can be carried on continuously.
2. Prior Art
A roll changer is a device used in conjunction with a winder to provide a continuous web winding operation. Roll changers sever the web of material when winding of one roll has been completed, and transfer the new incoming edge around the new core to start the new roll winding thereon. Existing roll changer designs are of two general types. In one, the roll change takes place at web speed. In the other, the roll changing takes place at zero web speed. In the case of zero web speed transfers, continuous web movement is provided by a web storage device known as an accumulator, which device is well known in the art.
Roll changers can additionally be classified as those which utilize adhesive on the core to start the new roll formation, and those which do not. Roll changers which do not utilize adhesive on the core are called enveloper type roll changers.
The introduction of new web materials and increasing operating line speeds has caused existing roll changer designs to produce objectionable defects in the roll changing sequence, and in some cases, the difficulties exceed the capabilities of the machines. For example, poor quality cuts and core starts with wrinkling and fold backs in the web material increase in occurrence as web speeds are increased, when conventional roll changing equipment is utilized. In addition, cutting problems in severing the web are more troublesome with some of the tougher materials now being introduced.
Most existing designs of roll changers utilize a rider roll which rides on the surface of the new core, with the web riding over a portion of the peripheral surface of the roll and then between the nip formed by the rider roll and the new core, and then moves around the periphery of the new core to the cutting station. The cutting station in many of these designs is not sufficiently close to the nip and thus a length of web which is greater than the distance from the cutting station to the nip is produced on cutting and thus insures a fold back of the web on the new core. Also, in these designs the blade is accelerated into the web to cause severance of the web.
Attached behind the knife and moving therewith is usually a brush which contacts the surface of the moving web immediately behind the severed edge and is intended to cause the web to be pressed against the new core. However, the brush follows the path of the knife and departs from the web just prior to the position where the severed web edge enters the nip between the rider roll and the new core. Also, unless the knife and brush are traveling at a substantially greater speed than the web, the brush is not able to travel along the web far enough to reach the severed edge before the edge reaches the nip, and thus the unsupported edge tends to fold back. This problem exists to a greater extent at higher web speeds since it is impractical to accelerate the knife and brush to a sufficiently high speed and then decelerate it, in the available space.
In addition, experience has shown that the problem of fold back is increased by problems of cutting. Cutting problems occur when the material is tough (i.e. resistant to tearing) or elastic, or when web speed increases. Problems caused by tough material are apparent in that the knife assembly must possess enough energy to penetrate the web and sever it. If the energy is too low, the cut does not occur or a uniform cut does not occur so that the edge of the web will not be straight and enter the nip between the rider roll and the new core at the same time across the width of the web.
The problem in cutting elastic material is in trying to maintain a consistent cutting position relative to the distance the edge must travel in order to enter the nip, so that too long a piece of web will not remain. Otherwise, the additional material will fold back on the new core as it enters the nip. The more elastic the web material, the later in the cutting stroke the cut occurs. This is due to the fact that the web behaves like a rubber band, and the knife cannot puncture it until enough resistance force builds up in the web.
The problems due to increase in web speed are not as apparent. The basic problem is the relative velocity between the web and the knife, as mentioned above. If web velocity exceeds knife velocity, the knife cannot penetrate the web; instead the web drags over the knife tip. When knife velocity is greater than web velocity, the blade penetrates the web and causes severance. The amount of speed differential causes the actual cut point to change in the same manner as occurs when a more elastic web is being cut. Any or all of the above problems can result in undesirable fold back along the surface of the core of the new roll, or even prevent severance of webs formed of certain materials.